Why does melting point decrease for impure solid?

In summary, the conversation discusses the concept of melting points and how they can be affected by the presence of other substances. The speaker suggests that the lower melting point of crude acetaminophen compared to pure acetaminophen is due to the substance it is mixed with having a lower melting point. They also mention how mixtures have higher entropy, which can decrease the driving force for melting. Additionally, the idea of intermolecular forces and how they can be weakened by the presence of other compounds is brought up as a possible explanation.
  • #1
erjkism
54
0
Okay, i am doing a lab involving crude acetaminophen and pure acetaminophen. The crude acetaminophen is supposed to have a lower melting point than the pure acetaminophen...

how does that work and why? does it have to do with thermodynamics or what??
 
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  • #2
The substance its mixed with may have a lower melting point; that's my guess anyway.
 
  • #3
Mixtures have higher entropy than pure substances. Because the transition from solid to liquid is mostly driven by the increase in entropy (ΔS = Sliquid - Ssolid), increasing Ssolid lowers the overall ΔS, lowering the driving force for melting. Since melting is less favorable, you need higher temperatures to accomplish it.
 
  • #4
ahhh... i see

thanks man
 
  • #5
You may also think in terms of intermolecular forces. In crystal molecules are packed in such a way that their interactions are strongest. When you add some other compound it stretches the crystal lattice, molecules are not in optimal positions and their interactions are weaker, thus less energy is required to break them apart.
 

1. Why does adding impurities decrease the melting point of a solid?

Adding impurities to a solid disrupts the regular arrangement of particles, making it more difficult for the solid to maintain its shape. This leads to a decrease in the amount of energy required for the solid to melt, resulting in a lower melting point.

2. How do impurities affect the melting point of a solid?

Impurities can lower the melting point of a solid by reducing the strength of the bonds between particles. This makes it easier for the solid to break apart and melt at a lower temperature.

3. Can impurities raise the melting point of a solid?

In some cases, impurities can actually raise the melting point of a solid. This is because certain impurities can form stronger bonds with the solid particles, making it more difficult for them to break apart and melt. However, this is not a common occurrence.

4. Is there a limit to how much impurity can be added before the melting point is affected?

No, there is no specific limit to how much impurity can be added before the melting point is affected. The amount of impurity needed to lower the melting point varies depending on the type of solid and the type of impurity being added.

5. How is the melting point affected by the type of impurity added?

The type of impurity added can greatly affect the melting point of a solid. Some impurities, such as those with a similar atomic structure to the solid, can have a larger impact on the melting point compared to others. Additionally, impurities with a higher concentration can also have a greater effect on the melting point.

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